1. Field of the Invention
The present invention relates to a solid imaging device incorporating charge coupled devices (CCD) or the like. In particular, the present invention relates to a solid imaging device of an interline transfer type suitable for the imaging of still images.
2. Description of the Related Art
Methods for obtaining high-resolution still images are generally known which employ an interline transfer type solid imaging device (e.g., a CCD) in combination with an optical shutter.
FIG. 5 is a schematic diagram illustrating two adjoining pixels 1a and 1b and a vertical transfer section 2 of a conventional interline transfer type solid imaging device. Specifically, FIG. 5 shows chronologically-occurring states ((a) to (e)) of the pixels 1a and 1b. The dots in FIG. 5 represent signal charges.
It is assumed that the pixel 1a constitutes a (2nxe2x88x921)th (or xe2x80x9codd-numbered xe2x80x9d) photosensitive section and that the pixel 1b constitutes a (2n)th (or xe2x80x9ceven-numbered xe2x80x9d) photosensitive section, where n is a natural number. The information in the odd-numbered photosensitive section 1a and the information in the even-numbered photosensitive section 1b together correspond to one bit in the vertical transfer section 2.
State (a) represents a state immediately after an optical shutter (not shown) has been shut following a period for obtaining sufficient exposure. It is assumed that the optical shutter remains shut throughout states (a) to (e).
In state (a), the photosensitive sections 1a and 1b store signal charges. In state (b), the signal charges in the odd-numbered photosensitive section 1a are all read into the vertical transfer section 2. In state (c), the signal charges which have been read into the vertical transfer section 2 are sequentially read to the outside, thereby providing a first field image signal corresponding to the odd-numbered photosensitive section 1a. 
Next, in state (d), the signal charges in the remaining even-numbered photosensitive section 1b are all read into the vertical transfer section 2. In state (e), the signal charges which have been read into the vertical transfer section 2 are sequentially read to the outside, thereby providing a second field image signal corresponding to the even-numbered photosensitive section 1b. 
FIG. 6 illustrates a plane pattern implementing the conventional photosensitive sections 1a and 1b and the vertical transfer section 2 schematically shown in FIG. 5.
In FIG. 6, broken lines depict lower electrodes 20; solid lines depict upper electrodes 21; a hatched portion represents a channel interruption region 5; and a dotted portion represents a surface channel region 4. In FIG. 6, the component elements which also appear in FIG. 5 are denoted by the same reference numerals as used therein.
The photosensitive sections 1a and 1b are formed on a low-concentration P layer. In general, the photosensitive sections 1a and 1b (PD) are implemented by using an embedded photodiode structure. The surface of the photosensitive sections 1a and 1b is a high-concentration P+ layer (FIG. 8A). The underlying one of the photosensitive sections 1a and 1b is an N layer (storage layer) for storing signal charges (electrons). The N storage layer is formed on the low-concentration P layer. The structure of the photosensitive sections 1a and 1b along the depth direction will be described in greater detail below with reference to FIG. 8B.
In general, the vertical transfer section 2 (VCCD) is implemented as an embedded channel composed essentially of an N surface layer overlying the low-concentration P layer. Clock signals xcfx86v2 and xcfx86v4 are applied to the lower electrodes 20. Clock signals xcfx86v1 and xcfx86v3 are applied to the upper electrodes 21.
FIG. 7 illustrates an operation of the interline transfer type solid imaging device shown in FIG. 6.
At time t0, an optical shutter (not shown) is shut after a period for obtaining sufficient exposure, corresponding to state (a) in FIG. 5.
During period t1, the vertical transfer section 2 is driven at a high speed so as to drain unnecessary charges within the vertical transfer section 2. At time t2, the signal charges in the odd-numbered photosensitive section 1a are read into the vertical transfer section 2, corresponding to state (b) in FIG. 5.
During period t4, the vertical transfer section 2 is driven at a normal speed so that the signal charges within the vertical transfer section 2 are read to the outside to give a first field image signal corresponding to the odd-numbered photosensitive section 1a, corresponding to state (c) in FIG. 5.
Next, at time t5, the signal charges in the even-numbered photosensitive section 1b are read into the vertical transfer section 2, corresponding to state (d) in FIG. 5.
Finally, during period t7, the vertical transfer section 2 is driven at a normal speed so that the signal charges within the vertical transfer section 2 are read to the outside to give a second field image signal corresponding to the even-numbered photosensitive section 1b, corresponding to state (e) in FIG. 5.
In recent years, a vertical overflow drain structure is generally adopted for the photosensitive sections 1a and 1b. 
FIG. 8A shows an exemplary vertical overflow drain structure. The vertical overflow drain structure shown in FIG. 8A includes a surface potential stabilization layer (high-concentration P+ layer) 13, a signal charge storage layer(N layer) 12, a potential barrier layer (low-concentration P layer) 11, and a substrate (low-concentration N layer) 10 underlying the potential barrier layer 11.
FIG. 8B is a graph illustrating a potential distribution of the vertical overflow drain structure shown in FIG. 8A.
As shown in FIG. 8B, if a large amount of signal charges are generated under excessive light and flow into the signal charge storage layer 12, a portion which cannot be stored in the layer 12 may overflow into the substrate 10. This is because the substrate 10 functions as a drain with respect to the signal charges so that the low-concentration P layer 11 forms a potential barrier. As a result, the signal charges will be stored up to a certain saturation value Qsat (defined below), past which the signal charges will be drained to the substrate 10.
The following problems may arise when the above-described vertical overflow drain structure is combined with an optical shutter.
For the sake of illustration, it is assumed that an amount (Qsat) of signal charges are stored in the signal charge storage layer 12 at time t0 in FIG. 7 (i.e., immediately after the optical shutter has been shut). The amount Qsat defines the saturation level, or the upper limit, of the amount of signal charges which can be stored in the signal charge storage layer 12.
Since no signal charges are generated in the photosensitive sections 1a and 1b between time to and the next time the shutter is opened, the saturation level Qsat of signal charges continuously decreases due to heat emission effects. This relationship between the saturation level Qsat and lapse of time can be expressed as follows (C. H. Sequin and M. F. Tompsett, translated by Takeishi and Kayama, xe2x80x9cCHARGE TRANSFER DEVICES xe2x80x9d, p.85, Kindai Kagakusha 1978):
Q(t)=Q0xe2x88x92Cxc2x7kTxc2x7In[1+(txe2x88x92t0)/xcfx84]xe2x80x83xe2x80x83eq. 1
In eq. 1, t and t0 represent points in time; C represents the capacity of the charge storage layer; k represents the Boltzmann constant; T represents absolute temperature; and xcfx84 represents a time constant which is determined in accordance with the structure of the charge storage layer.
The following values are illustrative of actual measurements that may be obtained in connection with the relationship shown in eq. 1:
Cxc2x7kT/Q0≈0.05, (txe2x88x92t0)/xcfx84≈1 to 500xe2x80x83xe2x80x83eq. 2
Accordingly, Q(t)/Q0≈0 to 0.3, indicating that Q(t) may be decreased by about 70% of the initial value.
FIG. 9 is a graph depicting the relationship of eq. 1. In accordance with the operation illustrated in FIG. 7, the Qsat level of the first field corresponding to the odd-numbered photosensitive section 1a is shown at Qa in the graph of FIG. 9 (assuming t2xe2x88x92t0=Ta). The Qsat level of the second field corresponding to the even-numbered photosensitive section 1b is shown at Qb in the graph of FIG. 9 (assuming t5xe2x88x92t0=Tb). Since Tb greater than Ta, it will be seen that the saturation signal for the second field is greatly reduced.
In order to obtain a high-resolution still image, it is necessary to construct a high-resolution still image, from two image signals, i.e., a first field and a second field. If the two signals have a large difference in saturation level, the saturation level for the entire image will be bottlenecked by the lower saturation level. This may result in a reduced dynamic range, which is a significant problem.
An interline transfer type solid imaging device according to the present invention includes a first photosensitive section, a second photosensitive section, and a vertical transfer section, the interline transfer type solid imaging device reading an image signal corresponding to a first field from the first photosensitive section and reading an image signal corresponding to a second field from the second photosensitive section. The interline transfer type solid imaging device operates as follows: (i) after effecting exposure for a predetermined period of time, light is shielded from entering the first and second photosensitive sections; (ii) first signal charges stored in the first photosensitive section are read into the vertical transfer section; (iii) a portion of second signal charges stored in the second photosensitive section are shifted into the first photosensitive section; (iv) the first signal charges in the vertical transfer section are read during a transfer operation; (v) the signal charges stored in the second photosensitive section and the signal charges stored in the first photosensitive section are read into the vertical transfer section, and added within the vertical transfer section so that the second signal charges are reproduced; and (vi) the reproduced second signal charges are read during a transfer operation.
In one embodiment of the invention, the first photosensitive section and the second photosensitive section each includes a signal charge storage layer, a substrate, and a potential barrier layer formed between the signal charge storage layer and the substrate, and the first photosensitive section and the second photosensitive section each have a vertical overflow drain structure for draining excessive signal charges stored in the signal charge storage layer to the substrate.
In another embodiment of the invention, the first photosensitive section and the second photosensitive section each include a high-concentration surface potential stabilization layer upon the signal charge storage layer, the potential stabilization layer having the opposite conductivity type from that of the signal charge storage layer.
In still another embodiment of the invention, a channel for transferring signal charges is provided between the second photosensitive section and the first photosensitive section.
In still another embodiment of the invention, the interline transfer type solid imaging device further includes: a lower electrode layer formed so as to extend between the first photosensitive section and the second photosensitive section; a channel region underlying the lower electrode layer for transferring signal charges between the first photosensitive section and the second photosensitive section; and a channel interruption layer formed so as to extend between at least a portion of the first and second photosensitive sections and the vertical transfer section, wherein a portion of the lower electrode layer covers a portion of the vertical transfer section.
In still another embodiment of the invention, the shifting of the port ion of the second signal charges from the second photosensitive section to the first photosensitive section is achieved by applying a read signal to the lower electrode layer.
In still another embodiment of the invention, the interline transfer type solid imaging device further includes: an upper electrode layer formed so as to cover a portion between the first photosensitive section and the first vertical transfer section, the portion between the second photosensitive section and the vertical transfer section, as well as a portion of the vertical transfer section, a channel region formed so as to extend between the first and second photosensitive sections and the vertical transfer section, under the upper electrode layer, a further vertical transfer section formed in a vicinity of the first and second photosensitive sections, and a further channel interruption layer formed between the further vertical transfer section and the first and second photosensitive sections.
In still another embodiment of the invention, the reading of the signal charges from the first and second photosensitive section into the vertical transfer section is achieved by applying a read signal to the upper electrode layer.
Thus, the invention described herein makes possible the advantage of providing a method and a structure for obtaining a high-resolution image signal by employing an interline transfer type solid imaging device in combination with an optical shutter, where the saturation level for a second field is prevented from lowering relative to the saturation level for a first field.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.